572 FRED M. UBER 



financial saving, particularly if there is any further reduction in the 

 conunercial prices. If one were to do so, it is not at all improbable 

 that a year or more of an investigator's research time would be thus 

 diverted. So far, the price of isotopic compounds, based on the 

 excess weight of the artificially concentrated isotope present, has 

 been quoted in terms of hundreds of dollars per gram. Since experi- 

 ments can often be made with quite small quantities of starting ma- 

 terial, the net cost per experiment is not really prohibitive. 



Governmental distribution of deuterium through the United 

 States Atomic Energy Commission (via 32) has recently been an- 

 nounced. It may also be obtained commercially for research pur- 

 poses (35) in the form of heavy water (D2O). N^^ has been made 

 available commercially (33) in concentrations as high as 60 atom per 

 cent excess. The N^^ occurs in the ammonium radical of ammonium 

 nitrate or in potassium phthalimide. Since relatively lower prices 

 prevail for the lower concentrations, it is more economical to order 

 the desired concentration initially than to obtain it later by direct 

 dilution. C^^ is also available {33) in concentrations as high as 60 

 atom per cent excess. No commercial source of stable sulfur isotopes 

 is known to the writer at present but separation methods have been 

 developed (/, p. 9). Limited quantities of isotopic oxygen can be 

 purchased through the Atomic Energy Commission (36). 



The number of synthetic organic compounds utilized in the past 

 has been limited by the nature of the starting materials available 

 and by the necessity of working on a micro scale. When stable 

 isotopes become more plentiful and cheaper, it will be possible to pur- 

 chase a wide variety of organic compounds that already contain one 

 or more isotopic labels in specified positions. One manufacturer has 

 already announced an intention to supply such compounds. 



2. Synthesis of Isotopic Compounds 



For the most part the ordinary synthetic methods of organic 

 chemistry are utilized for the preparation of compounds containing 

 isotope tracers. There is often a change in emphasis, however, such 

 as the calculation of the yield on the basis of the isotopic starting 

 compound rather than on some less expensive but more customary 

 raw material. In synthesizing amino acids, for example, the crucial 

 yield is the one based on ammonia since it contains N^^, rather than 

 the yield related to the initial carbon chain. For this reason some 

 N^^ amino acids have been prepared more economically by the 



